The transcription factor Hoxa 13 plays an essential role as a regulator of genitourinary (GU) growth and development. In humans, mutations in HOXA 13 cause Hand Foot Genital (HFGS) and Guttmacher (GS) Syndromes, two autosomal dominant disorders that profoundly affect the development of the limb, bladder, ureter, uterus, and external genitalia. In mice, the GU function(s) of Hoxa 13 are conserved, as mutations in Hoxa 13 also cause malformations of the bladder, ureter, uterus, and external genitalia. What is intriguing about the GU malformations associated with the loss of Hoxa 13 function is that many of these defects reflect changes in the patterning of urogenital sinus, a developmental region from which the external genitalia, uterus, and most of the bladder are derived. More importantly, while the developmental origins of the bladder, uterus, and ureters are well understood, surprisingly little is known about the cellular and molecular signals required for the normal ontogeny of these structures. Recently, we demonstrated that Hoxa 13 deficient mice could be used to elucidate the molecular and cellular mechanisms required for normal development of the external genitalia. Here hypospadia associated with loss of Hoxa 13 function was determined to be caused by the loss of Fgf-8 and Bmp7 expression in the urogenital sinus and urethral plate epithelium. At the cellular level this loss in growth factor signaling directly affected cell proliferation and apoptosis, causing defects in the closure of ventral urethra as well as malformation of the excretory meatus. Recognizing that mutations in Hoxa 13 also affect the formation of the bladder and ureter, I hypothesize that many of the cellular and molecular mechanisms required for the formation of these structures can be elucidated by examining how loss of Hoxa 13 function impacts cell signaling, gene expression, and developmental patterning of these affected structures. To test this hypothesis, cells expressing a mutant Hoxa13-GFP allele will be purified from the bladder and ureter to identify changes in gene expression. Next the cis-acting DNA regulatory elements bound by Hoxa 13 that direct the tissue-specific expression of affected target genes will be identified. Sequence comparisons of these cis-acting elements will be performed to identify which DNA sequences function as bladder or ureter-specific regulators of gene expression. These candidate cis-acting elements directing Cre recombinase expression will be tested in transgenic embryos to evaluate their capacity to mutate genes in a bladder or ureter-specific manner, providing new resources to selectively regulate the tissue-specific function of genes in the GU region.